1. Field of the Invention
Embodiments of the present disclosure relate to purification of carbon nanomaterials and, in particular, to systems and methods for the removal of impurities from carbon nanotubes.
2. Description of the Related Art
Carbon nanotubes (CNTs) hold significant promise for use in the development of new technologies for national defense and consumer industries. This promise owes in part to the unique combination of nanoscale dimensions and superior chemical and physical properties exhibited by CNTs. Notably, advancements in the carbon nanotube field in the last decade owe much to the progress made in the purification of single-walled carbon nanotubes (SWNTs), as cutting edge technologies benefit significantly from relatively pure materials. As such, efforts to improve the quality of SWNTs are ongoing.
One hindrance to this goal is that the production of SWNTs, and CNTs in general, generate nanotubes that are contaminated with various impurities, such as catalysts, amorphous carbon (AC), and graphitic nanoparticles (GNP). Efficient procedures for the purification of SWNTs have been developed for use in producing small quantities of purified SWNTs for use in laboratories but adapting these techniques to industrial mass production remains problematic. For example, while SWNTs may be treated with nitric acid in order to take catalyst residues into solution and remove them from the SWNTs, the nitric acid also reacts with carbonaceous materials present in the CNTs, producing additional amorphous carbon.
Further purification steps may also be hampered by similarities between the chemical properties of carbon nanotubes and the chemical properties of carbonaceous impurities, as well as the stability of such impurities. In one example, carbon nanoparticle impurities may comprise another form of carbon, such as graphitized carbon, with one or more dangling bonds. As a result of this similarity, it is difficult to chemically attack the impurities without also damaging or destroying the CNTs. Furthermore, some of these impurities have a higher decomposition temperature than nanotubes, limiting the utility of thermal purification techniques. Thus, fabrication of high-purity CNTs necessitates the use of expensive, multi-step purification processes that are ill-adapted and cost prohibitive for industrial scale production.